Seismic geophysical surveys are often used in the oil and gas industry in order to map stratigraphy of subterranean formations, lateral continuity of geologic layers, locations of buried paleochannels, positions of faults in sedimentary layers, basement topography, and various other geological structures. The resulting maps are typically deduced through analysis of the nature of reflections and refractions of generated seismic waves from interfaces between the multiple layers within the particular subterranean formation being mapped.
Seismic activity generally emits elastic waves in the form of compressional waves (“P-waves”) and shear waves (“S-waves”). The generated P- and S-waves travel through the surrounding earth and are reflected by various subterranean formations to be detected by an adjacent detection system comprising, for example, an array of seismic detection devices, receivers, or “geophones.” As the P- and S-waves reach the detection system, the seismic detection devices transduce the P- and S-waves into representative electrical signals that are analyzed to determine the seismic nature of the subterranean formations at the given site.
Historically, the receivers have been placed at the surface. More recently, however, borehole seismology has been undertaken by positioning the receivers in a well borehole. The data collected from the receivers along the borehole is known as a vertical seismic profile (VSP). VSP methods advantageously allow for increased seismic frequency content, which provides greater detail of the geophysical features.
A walkaround VSP can provide various seismic attributes to characterize fractured rocks or stress fields around a well. Direct P-wave arrival time and particle motion polarization extracted from three-component (3-C) data that is recorded by three orthogonal geophones can provide an understanding of fracture or stress-induced azimuthal velocity anisotropy in a depth interval over the receiver array. Consider an ideal situation where a studied area is a horizontally layered medium, the well is vertical, and shots are placed on a circle with a constant offset, which is defined as the horizontal distance between the surface source and the downhole receiver. The measured travel time or the travel time difference over the receiver array, referred to as local slowness, is used to detect and estimate the azimuthal anisotropy.
In a land walkaround survey, multiple shot points are placed about the opening to the well at a constant radial offset from the well. However, land walkaround surveys can be constrained by geographical/geological restrictions (e.g., lack of suitable terrain to place the shots), permit restrictions (e.g., lack of availability of necessary governmental permits to conduct the surveys), and the like, and it may thus not be possible to place all shot points with the same radial offset from the opening to the well. As a result, some shot points may be positioned at different radial offsets, or it may be required to exclude some shot points from the survey. In addition, unconventional shale plays can have vertical transverse isotropy (VTI), also referred to as polar anisotropy. As both azimuthal anisotropy and polar anisotropy are dependent on shot offsets (or polar angles), irregularly placed shot points make it difficult to reveal and quantify the azimuthal anisotropy from the P-wave data.